The G protein-coupled receptor (GPCR) CXCR4 is a determinant of the pathogenesis of HIV-1 infection and breast cancer, but efforts to develop drugs that modify its function have been unsuccessful. Traditional assays that screen GPCR function mostly use live cells or cell membrane fragments, formats that are limited by poor receptor purity, stringent environmental requirements, unacceptable variability, and an inability to be miniaturized, prohibiting their application to more versatile micro- and nano-scale detection technologies. Furthermore, traditional assays for GPCR activation often involve detection of downstream signaling events such as calcium release which are not stimulated by a number of important GPCRs and G proteins. A nanometer-scale, cell-free assay for GPCR activation that can be applied to microfluidic drug-screening devices could have a major impact on the discovery of drugs to difficult GPCR targets, and this proposal is designed to produce such a system. Our approach has three major advantages compared with existing assays: 1) simplicity, 2) the ability to be miniaturized to the nanometer scale, and 3) the ability to detect diverse and difficult GPCRs that conventional systems fail to detect. The product will facilitate high throughput screening of GPCRs, orphan ligand-receptor pairing, and the development of therapeutics that treat AIDS and breast cancer.